Relating to an arm bone (5)
I believe the answer is:
ulnar
'relating to an arm bone' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ulnar that I've seen before include "Relating to a particular arm bone" , "Relating to one of the bones of the forearm" , "Relating to a bone in the forearm" , "of certain bones" .)